Two nights of phase-resolved medium resolution VLT spectroscopy of the extra-galactic low mass X-ray binary LMC X-2 have revealed a 0.32 \pm 0.02 day spectroscopic period in the radial velocity curve of the He II \lambda 4686 emission line that we interpret as the orbital period .
However , similar to previous findings , this radial velocity curve shows a longer term variation that is most likely due to the presence of a precessing accretion disk in LMC X-2 .
This is strengthened by He II \lambda 4686 Doppler maps that show a bright spot that is moving from night to night .
Furthermore , we detect narrow emission lines in the Bowen region of LMC X-2 , with a velocity of K _ { em } =351 \pm 28 km s ^ { -1 } , that we tentatively interpret as coming from the irradiated side of the donor star .
Since K _ { em } must be smaller than K _ { 2 } , this leads to the first upper-limit on the mass function of LMC X-2 of f ( M _ { 1 } ) \geq 0.86 M _ { \odot } ( 95 % confidence ) , and the first constraints on its system parameters .